Acoustic emission crack monitor

ABSTRACT

A system for monitoring crack growth in a structure in which acoustic energy released as a result of a crack event is sensed and integrated to provide a binary signal representative of the number and intensity of individual crack events. The binary crack energy data signals are accumulated and a warning device is activated when the stored crack data exceeds a predetermined level. Means may be provided to discriminate crack event signals from similar acoustic signals resulting from noise and impact or vibration.

United States Patent 1 1 1 3,713,127 Keledy et al. 1 Jan. 23, 1973ACOUSTIC EMISSION CRACK R. Notvest, Ramsey, both of NJ.

OTHER PUBLICATIONS Dunegin & Harris, Ultrasonics, 7/69, pp. 160-161.

Primary ExaminerDonald J. Yusko [73] Assignee: Trodyne Corporation,Teterboro, Assistant Examiner-Michael Slobasky NJ. At1orneySandoe,Hopgood & Calimafde [22] Filed: Oct. 16, 1970 [57] ABSTRACT [21] Appl.No.: 81,448

A system for monitoring crack growth in a structure in 3 2 l 3 6 3 67 3which acoustic energy released as a result of a crack 40/ 6 event issensed and integrated to provide a binary 58] Fie'ld 3 69 67 4 signalrepresentative of the number and intensity of in- I dividual crackevents. The binary crack energy data [56] References a signals areaccumulated and a warning device is activated when the stored crack dataexceeds a predeter- UNITED STATES PATENTS mined level. Means may beprovided to discriminate 3,543,261 11 1970 Burney ..340/261 crack eventsignals from similar acoustic Signals result- 3,364,477 1/1968 McDonough..340/261 ing from noise and impact or vibration. 2,788,659 4/1957Radnar et al. ..73/67.4 3,585,581 6/1971 Anne et al. ..340/261 18Claims, 3 Drawing Figures p/szo f/VANSDl/Cf'l? PEAK a: :crm AFM 45 541.544% 41mm; -rafire-gimp 057567? 60%23'22 .72 mew? fNME/D/J'AELED/SL'l/M/NAWQ wit Z5 mm r g 26140007 PATENTED JAN 23 1975 SHEET 2 BF 24? ATIURNEG' ACOUSTIC EMISSION CRACK MONITOR The present inventionrelates to crack detecting apparatus, and more particularly to a crackdetecting and monitoring system which provides a real-time indication ofcrack growth in a material.

Cracks are formed in a material as a result of factors including fatigueor corrosion in the material. If the developing cracks remain undetectedand are allowed to increase in area, the crack may reach sufficient sizeto cause the material to fail at stresses below the normal yieldstrength of the material. This is believed to result from the resultingconcentration of localized stresses above the yield strength at thelocation of the crack in the material. The larger the crack, the greateris the likelihood of brittle failure in the material.

The need thus exists for apparatus capable of detecting the presence andthe growth of such cracks in a material, and providing an indication ofpossible material failure long before the occurrence of a failure, whichmay have catastrophic results. An additional benefit derived from themonitoring of the crack history of a structure is that it permits anaccurate analysis of the metallurgical history of the crack-plaguedstructure, thereby enabling the metallurgist to often eliminate thecauses of the cracking. Cracking due to interstitial embrittlement,stress-rupture, transformation cracking, stress corrosion, and delayedcracking may be thus identified and prevented.

It has been observed that materials when undergoing fracture produce awave-like propagation of released strain energy (stress wave emission)also known as acoustic waves. In the formation of cracks in thematerial, particularly those resulting from fatigue, it has beenobserved that the crack growth is incremental, that is, the crack growsin discrete steps rather than in a continuous manner. Upon eachincremental crack event, a burst or pocket of acoustic energy isreleased and propagated in the material. The energy released is ofvarying amplitude corresponding to the magnitude of the crack event, andis at random frequencies extending over a broad band spectrum.

It has been proposed to detect these crack generated acoustic waves(hereinafter also referred to as crack energy or crack event signals) asa means for sensing and monitoring the formation and growth of cracks.These proposed systems have, however, been hampered by their failure todistinguish the crack energy and external interference and noise, aswell as by their inability to relate the acoustic crack signals to arealtime indication of the progress and extent of the crack.

It is an object of the invention to provide an instrument which reliablyand accurately indicates the extent of crack growth in a structure.

It is a further object ofthe invention to provide a crack detectioninstrument of the type described which is able to discriminate betweencrack energy and mechanicalnoise energy.

It is another object of the invention to provide a crack detection andmonitoring instrument having the capability of automatically providingan alarm when the monitored crack growth has exceeded a predeterminedsafe level. It is yet a further object of the present invention toprovide a crack detection and measuring instrument capable of providinga real-time indication of the crack growth history of the structurebeing monitored.

To these ends, the crack detection instrument of the invention comprisesmeans for detecting the release of crack energy and for thereafterintegrating the de tected crack energy signals, and converting theintegrated signals into a binary count signal which is representative ofthe number and intensity of the individual detected crack events. Thecrack data count signals are accumulated to provide a real-timeindication of the extent of crack growth in the material. Whenever theaccumulated crack data exceeds a predetermined safe level an indicatoris actuated to thereby automatically provide a warning signal.

The crack energy sensor is herein shown as a piezoelectric transducerelement mechanically attached to the structure being monitored. Thesignal produced by that element upon the propagation of crack or otheracoustic energy is passed through an active filter which filters outnormal vibration and acoustic noise. The amplified signal is thenapplied to a level detector where it is compared against a preset levelabove the noise level at that stage. As a result only significant levelacoustic signals are processed to derive the binary crack event signalswhich are thereafter stored.

In another aspect of the invention, means are provided to distinguishbetween crack energy acoustic signals and acoustic signals resultingfrom mechanical impact on the structure being monitored. To this end,the instrument of the invention may be provided with an impactdiscriminator which operates by analyzing the waveform of the detectedacoustic signal and then comparing that signal against a syntheticwaveform. In the event that a spurious impact signal is sensed, theoperation of the count signal generating circuit is disabled for theduration of the impact signal.

To the accomplishment of the above and to such further objects as mayhereinafter appear, the present invention relates to an acousticemission crack monitor substantially as defined in the appended claims,and as described in the following specification taken together with theaccompanying drawings in which:

FIG. 1 is a schematic block diagram of the crack detection instrument ofthe invention;

FIG. 2 is a more detailed schematic diagram of the instrument of FIG. 1;and

FIG. 3 is a cross-sectional view of a typical holder for mounting theacoustic energy detector employed in the instrument of the invention onthe structure to be monitored for crack growth.

The crack detection instrument illustrated in the drawings has thecapability of detecting and distinguishing the occurrence ofa crackevent in a structure, by sensing the release of acoustic energy from thestructure. The crack event data is integrated and then converted tobinary form representative or the number and the intensity of theindividual crack events. The stored crack event data can be employed toprovide a warning signal whenever the stored crack data exceeds apredetermined safe level. I

Referring to FIG. 1, the instrument of the invention illustrated thereincomprises an acoustic energy sensor in the form of a piezoelectrictransducer 10, such as a 1.7 mHz frequency ceramic piezo. As describedmore completely with reference to FIG. 3, transducer 10 is mounted in arelatively massive holder that is adapted to be clamped to the structurebeing monitored in a manner such that the transducer is sensitive toshear waves that may occur in the structure such as a result of a growthof a crack (crack event). The natural frequency of the transducer holderis independent of that of the transducer piezo element and issufficiently low such that any extraneous signals resulting from holdervibration are electronically filtered by the transducer.

The electrical signals produced by transducer 10, as a result of therelease of acoustic energy in the surface, are coupled to apre-amplifier 12, which is preferably, and as shown in FIG. 3, fixedlymounted on the transducer holder. The mounting of pre-amplifier 12 inthis manner serves to reject common mode spurious signals and to providea low impedance source to the following stages of the instrument.

The amplified signals from pre-amplifier 12 are coupled to afilter-amplifier 14 which includes a pass-band filter for passingsignals at frequencies centered about 200 kHz, to thereby eliminateextraneous noise signals from the detected acoustic signal. Theamplified signal at the single ended output of amplifier 14 is an analogsignal that is coupled to a peak detector and integrator 16. Detector l6detects the envelope of the amplified transducer signal and convertsthat signal into a peak signal proportional to the amplitude of theanalog crack event signal.

The peak signal output of peak detector 16 is applied to a leveldetector 18 at which the peak signal is compared to a preset referencesignal. The level of the reference signal is determined by the level ofthe noise voltage at the level comparator in detector 18 such that leveldetector 18 produces no output level signal as a result of only a noisevoltage signal.

When the peak signal exceeds the reference signal, detector 18 producesa level signal which has a period proportional to the amplitude of thepeak signal for reasons to be described below with reference to theschematic diagram of FIG. 2. Thus, as desired, detector 18 produces anoutput level signal only upon the detection of a significant acousticenergy release from the structure being monitored.

The level signal is applied to a normally disabled analog-to-digitalconverter 20, which produces one or more crack event pulses upon thepresence of a level signal at the output of level detector 18 and theabsence of a disabling pulse, as will be described below. The number ofcount pulses produced by converter 20 is proportional to the duration ofthe level signal, and is thus proportional to the intensity of thedetected crack event signal. The count signals are applied to a pulsedrive circuit 22 which converts the count pulses into drive pulses whichare in turn accumulated or stored in a counter 24 here shown in the formof an electromechanical counter which is preset to a count correspondingto a maximum safe crack level or area in the structure. When thecumulative count in counter 24 exceeds the preset count a warningindicator 26 is actuated to indicate an unsafe crack condition in thestructure. Counter 24 may also be periodically interrogated to determinefrom its stored count, the extent of crack growth in the structure sothat a prediction of the remaining useful life of the structure can bemade, and appropriate maintenance procedures can be 7 developed andinitiated.

ln another aspect of the invention, the crack detection instrument isfurther provided with an impact dis criminator 32 which compares, in amanner more completely set forth in a later part of the application, thepeak signal and a reference synthetic signal derived from the peaksignal. In the event that discriminator 32 determines that the levelsignal is a result of an impact energy signal rather than a crack energysignal, it produces an inhibit signal which disables analog-to-digitalconverter 20 for the complete duration of the level signal, and thecumulative count in counter 24 remains unchanged during this period.

The system illustrated in block form is illustrated in greater detail inFIG. 2 in which corresponding sections are designated by the referencenumerals employed in FIG. 1. As shown in FIG. 2 the upper and lowerterminals 34 and 36 of the piezoelectric transducer 10 are respectivelyapplied to the gate terminals of PET Q1 and PET Q2 which define thedifferential pre-amplifier 12. The output terminals of FETs Q1 and Q2are coupled through capacitors Cl and C2 to the input terminals of anoperational amplifier 38, the output of which is in turn coupled througha network 40 comprising series-connected capacitors C3 and C4 andresistors R1 and R2 connected to these capacitors and ground, to theinput terminal of a second operational amplifier 42. Amplifiers 38 and42 and network 40 define active amplifier 14 which amplifies and filtersthe acoustic energy signal about a pass band of 200 kHz to remove normalvibration and acoustic noise.

The single-ended output of amplifier 42, which is in the form of ananalog signal having an amplitude proportional to the magnitude of thedetected acoustic energy, is applied through a capacitor C5 to the inputof an amplifier 44. A diode D1 is connected to the output of amplifier44 and to a point 46 to which one terminal of a capacitor C6 isconnected. The other terminal of capacitor C6 is connected to ground.Point 46 is also connected to the other input of amplifier 44.

Amplifier 44, diode D1, and capacitor C6 define the peak detector andintegrator 16 and serve to detect the envelope of the analog signal andto charge capacitor C6 to a peak signal proportional to the envelopeamplitude. The peak signal is applied to one input of acomparator-amplifier 48, the other input of which is a preset referencesignal obtained at the wiper arm 50 of a variable resistor R3. Acapacitor C7 is connected between wiper arm 50 and ground.

Amplifier-comparator 48 defines level comparator l8 and produces a levelsignal in the form of a pulse whenever the peak signal exceeds thepreset reference signal. The level of the latter is preset by a suitableadjustment of resistor R3 in accordance with the ambient mechanicalnoise signal at the other comparator input, with the desired result thatno level detection is made at comparator 48 under noise signals only.

During the operation of comparator 48, the peak signal stored oncapacitor C6 discharges through comparator 48 and capacitor C7. Thecomparator will continue to produce the level signal until the peaksignal has discharged to the level of the reference signal. As a resultthe duration of the level signal pulse is proportional to the amplitudeof the peak signal, and thus to the magnitude of the sensed acousticenergy signal.

The level signal produced by level detector 16 at the output ofcomparator 48 is applied through a coupling network consisting of acapacitor C8 connected in parallel with a resistor R4 to the base of atransistor Q3 connected in a grounded emitter configuration. The levelsignal is inverted by transistor Q3 and the thus inverted signal iscoupled to a one-shot 50 which, when triggered by the inverted levelpulse, generates a 125;}. s strobe gate. The gate is employed in amanner which is described in a later part of the specification inconjunction with the operation of impact discriminator 32.

The level signal is also applied to one input of a NOR gate 52 theoutput of which is in turn connected to the input of a NOR gate 54. Inthe absence of an inhibit signal at the other input of gate 52 (which isproduced in response to impact acoustic energy signals, as will bedescribed below) an enabling signal is produced at the output of gate54. The enabling signal is applied through a resistor R5 and a diode D2to the emitter ofa unijunction transistor Q4. A capacitor C9 isconnected between the emitter of transistor Q4 and operating voltage isapplied to that emitter through a resistor R6. The bases of transistorQ4 are respectively coupled to the voltage supply through a resistor R7and to ground.

Transistor Q4 defines a free-running sawtooth oscillator that isoperable only in the presence of the enabling signal at its emitter; inthe absence of that signal the oscillator is inoperative. When theenabling signal is present, the oscillator produces a clock signal at a1 kHz rate. The clock signals are coupled through a capacitor C10 to adecade counter 56 which produces a count signal at its output for eachseries of ten input clock pulses. Since the oscillator is only enabledfor the period of the enabling signal, the number of count pulsesreceived at counter 56 is thus representative of the duration of thelevel signal and thus of the amplitude of the detected acoustic energysignal. The oscillator, comprising transistor Q4, and counter 56 thusdefines the analog-to-digital converter 20.

The binary count signal or signals produced by counter 56 are applied toan inverting NOR gate 58, the output of which is in turn'coupled to anOR gate 60. The output of the latter gate is in turn coupled to oneinput of a NOR gate 62 whose output is in turn coupled to the otherinput of gate 60.

Gates 60 and 62 thus define a count signal latching circuit. The pulseoutput of that latching circuit is coupled to the trigger input of aone-shot 64 which, upon the receipt of a trigger pulse, generates a msdrive signal. The low output of one-shot 64 is coupled through a diodeD3 to a timing circuit consisting of a resistor R8 and a capacitor C11to an inverting NOR gate 66 which has an output connected to the inputof the latch circuit NOR gate 62. The timing circuit is charged duringthe 20ms on" period of oneshot 64, and at the end of thatperioddischarges to produce a ms disabling signal to the latch circuit.Should there be an additional count signal received from counter 56during the combined on-off ms period of one-shot 64, that signal will beheld in the latch circuit and applied to the one-shot at the completionof the 30ms off period to produce a new drive 20ms signal at the highoutput at one-shot 64.

The drive signal output of one-shot 64 is coupled through a resistor R9to the base of a transistor Q5. The

emitter of transistor O5 is connected to the base of a transistor Q6connected in a Darlington pair configuration with transistor Q5. Thecollector of transistor O6 is connected to a device for accumulating orstoring the individual crack event drive signals, here shown as amechanical counter 68 represented by a coil 70 across which is connecteda diode D4.

Counter 68 is provided with a preset count representing the maximumnumber of crack events that can be tolerated in the structure beingmonitored within the limits of safety. Each drive signal applied to thecounter causes its count to be updated by one so that the cumulativecount of counter 68 is directly representative of the number andrelative intensities of the individual detected crack even-acousticsignals, since, as will be understood from the above, a single crackevent of a relatively high intensity can produce two or more drivesignals to the crack event signal storage counter.

When the cumulative crack event count equals the preset threshold count,a mechanical switch 72 is actuated causing the normally open contacts 74to close, and the normally closed contacts 76 to open. The formeroperation causes an indicator, here shown as a lamp 78, to be energizedto thereby provide an alarm that the structure under crack surveillancehas been weakened to a critical extent as a result of continuing crackactivity. The relay contacts may also control external indicators (notshown) that can be connected at terminals 80 and 82.

In order for the warning of indicator 78 to be independent of detectedacoustic signals such as those that are caused by mechanical impacts onthe structure, the instrument of the invention also includes, as notedabove, impact discriminator 32. Discriminator 32 operates by a signatureanalysis on the detected analog waveform, based on the observation thatmost crack induced acoustic energy signals have a relatively rapid orsharp rise time, while impact induced signals have a relatively slowrise time in excess of 125 1.5.

Discriminator 32 comprises amplifiers 84 and 86, each of which receivesthe analog inputs from amplifier 42 at one of its inputs. The otherinputs of amplifiers 84 and 86 are coupled through a resistor R10 to apoint 88, and the outputs of these amplifiers are connected in commonthrough a diode D5 to point 88. A capacitor C12 is connected betweenpoint 88 and ground.

The 125 ps strobe signal produced by one-shot 50 as described above isapplied through a resistor R11 to a control terminal of amplifier 86 andcauses that amplifier to be operative only during the strobe period.Amplifiers 84 and 86 thus integrate and produce a peak signal comparableto that produced at point 46 of peak detector 16, but only during thestrobe period. The peak signal at point 46 is referred to as the realpeak, while the peak signal at point 88 is referred to as the syntheticpeak.

The real and synthetic peaks are applied respectively to the two inputsof a comparator 90, the former through a resistor R12. A preset inhibitbias signal derived from a variable resistor R13 is coupled through aresistor R14 to the input of comparator 90 which also receives thesynthetic peak. The output of comparator 90 is connected through aresistor R15-to a point 92. The strobe signal is also applied to point92 through a diode D6 to unconditionally maintain point 92 high duringthe strobe period. A capacitor C13 is connected between point 92 andground.

Point 92 is also connected to an input of a NOR gate 94 which has anoutput connected to one input of a NOR gate 96, the output of which iscoupled back to the other input of gate 94. The other input to gate 96is the level signal output of comparator 48 and its output is coupled tothe input of gate 52, it being recalled that the other input of gate 52also receives the level signal.

In operation, comparator 90 compares the real and biased synthetic peaksat a time approximately 125;; s after the production of the level signalat comparator 48, that is, the detection of a significant acousticenergy signal. During the initial 125g period, the output of comparator90 is high and the enabling signal is produced at the output of gate 54.However, the sawtooth oscillator which operates at a 1 kHz rate is notactuated by the level signal during this initial strobe period. At theend of the strobe period, point 92 is no longer unconditionally chargedhigh, amplifier 86 is disabled, and the synthetic peak signal havingreached a maximum level at the end of the strobe period now begins todecrease.

For a crack event signal, as opposed to an impact signal, the real peaksignal at the end of the 125p. s strobe period has decreased from itsinitial high level, so that the level of the synthetic peak at that timeexceeds that of the real peak and the output of comparator 90 istherefore low. On the other hand, an impact signal produces a real peaksignal which is still increasing in level after the end of the strobeperiod, and would thus exceed the level of the synthetic peak signal atthat time. For this latter impact condition of the real and syntheticpeak signals, comparator 90 produces a high or inhibit signal at point92 which is applied to one input of gate 94. That signal, when producedin the manner just described, is coupled in the logic and latching gates52, 54, 94 and 96 to cancel the enabling signal and thus disable theclock pulse oscillator. As a result of the operation of the latchcircuit consisting of gates 94 and 96, the impact signal once detectedin discriminator 32 will disable the clock oscillator for the durationof the event, that is, the duration of the level signal. As a result theclock oscillator produces crack event count pulses only upon thedetecting and distinguishing of a crack event acoustic signal, as isdesired, and the instrument of the invention thus has the ability todistinguish between crack acoustic energy and acoustic energy resultingfrom mechanical impact.

FIG. 3 illustrates a holder assembly that is highly suited for mountingthe acoustic transducer element to the structure being monitored. Theholder, which preferably has a mass significantly larger than that ofthe transducer element 10, is yoke-shaped and includes a base 98 fromwhich two arms 100 and 102 extend, a recess 104 being defined betweenarms 100 and 102. Arm 100 has a threaded bore formed therein in which ananvil 106 is securely received. Piezo transducer is mounted on anvil 106and insulated therefrom by an insulating bushing 108. Printed circuitboards 110 and 112 containing FETs Q1 and Q2 and the associatedresistances in the two sections of the differential pre-amplifier 12 aresecurely mounted on arm 100. As noted above, FETs Q1 and Q2 areconnected to transducer 10 by the electrodes 34 and 36 extending fromand connected to the opposing surfaces of the piezo element, as shown inFIG. 3.

Threaded receptacles 114 (only one of which is seen in FIG. 3) provideelectrical connection between printed circuit boards and 112 and theexternal active filter 14, and the remaining sections of the crackdetection instrument. A threaded slot 116 is formed in arm 102 andreceives a set screw 118.

In use, the structure under crack surveillance is placed within recess104 and the set screw 118 is turned until the transducer holder isfirmly clamped against the structure, in a manner enabling the detectionby transducer 10 of acoustic energy released and propagated through thestructure.

The crack detection and monitoring of the invention thus provides anaccurate and reliable means for detecting the occurrence and cumulativeeffect of crack events occurring in a structure under cracksurveillance, and moreover, automatically provides a warning signal whenthe cumulative crack data indicates an amount of crack activityexceeding a predetermined safe level.

Tests performed on one practical embodiment of the instrument of theinvention have indicated a substantially linear relation between theextent of crack growth and the readout of the crack event counter 24over a wide range of stress intensity and fracture toughness of thematerial of the structure being monitored. Moreover, the system of theinvention is relatively insensitive to noise by virtue of low frequencyattenuation and impact discrimination. In one practical embodiment ofthe invention crack areas as low as 0.0001 in. and crack increments aslow as 0.0004 in. were detected, and crack data of 0.01 in. wereaccumulated.

The crack detection and monitoring instrument of the invention can thusbe used to great advantage in preventive maintenance operations as wellas in metallurgical analysis on the crack susceptibility of structuresunder predetermined conditions such as stress and fatigue. Theinstrument of the invention may also be advantageously employed withrespect to thermally hot structures, such as weldments and structuresbeing heat-treated. In this manner cracking such as temper embrittlementand stress rupture can be monitored relative to the time-temperatureparameters causing the cracking.

While the crack event counter is herein shown as a mechanical counter,binary system such as a storage shift register or a binary accumulatorcould be employed to equal advantage whenever higher operating speedsare desired. Moreover, instead of providing an instantaneous, real-timeindication whenever the stored crack data exceeds a predetermined levelas herein specifically disclosed, the binary count data could also bestored in a suitable binary storage device which can be periodicallyinterrogated for an indication of the level of crack growth.

Moreover, the instrument of the invention need not be limited toexclusively provide real-time indication or a stored crack data, but maybe adapted to includeboth data storage arrangements in a single crackdetection instrument and system.

Thus, while only a single embodiment of the present invention has beenherein specifically described, it will be apparent that modificationsmay be therein without departing from the spirit and the scope of theinven tion.

We claim;

1. Apparatus for detecting and monitoring the incremental growth of acrack in a structure, comprising means for detecting the release ofacoustic energy from the structure such as that produced as a result ofa crack event in the structure, said detecting means ineluding means forproducing an analog signal having a predetermined relation to themagnitude of the energy released upon each release of such energy, andmeans coupled to said analog signal producing means for deriving abinary signal representative of the number and amplitude of said analogsignals, said binary signal deriving means including means for producingone or more count signals the number of which bears a predeterminedrelationship to the amplitude of said analog signal, means coupled tosaid count signal producing means for accumulating said count signals,and means coupled to said detecting means for sensing the nature of thedetected analog signal between a crack event signal and an impactsignal, and means coupled to said sensing means for disabling saidbinary signal deriving means upon the determination that said detectedanalog signal is a result of an impact on the structure.

2. The apparatus of claim 1, further comprising means for comparing thenumber of accumulated count signals against a preset reference level,alarm means, and means coupled to said comparing means for actuatingsaid alarm means when the number of accumulated counted signals equalsor exceeds said reference level. i 3. The apparatus of claim 1, furthercomprising means for operatively comparing said analog signal and apreset reference level and for producing an enabling signal when theformer bears a predetermined relation to the latter.

4. The apparatus of claim 3, further comprising means operativelyinterposed between said detecting means and said comparing means foroperatively integrating said detected analog signal to thereby produce apeak signal, said peak signal being applied to one input of saidcomparing means.

5. The apparatus of claim 3, in which said count signal producing meanscomprises pulse producing means coupled to said comparing means andactuated in response to said comparing means when said analog signalbears said predetermined relation to said reference signal.

6. The apparatus of claim 5, further comprising means coupled to saidpulse producing means for summing the pulses produced by the latter,second means for comparing the summed number of said count signalsagainst a preset count, and means for producing a warning signal whenthe former equals or exceeds the latter.

7. The apparatus of claim 1, further comprising means for deriving asynthetic signal from said detected analog signal, said sensing meanscomprising means for comparing the detected analog signal and saidsynthetic signal at a predetermined time after the detection of saidanalog signal and for producing a count disabling signal when the formerbears a predetermined relationship to the latter.

8. The apparatus of dam 7, in which said synthetic signal deriving meanscomprises second detecting means receiving said analog signal and meansfor enabling said second detecting means after said analog signaldetection and for thereafter disabling said second detecting means atsaid predetermined time thereafter.

9. The apparatus of claim 8, in which said enabling and disabling meanscomprising means coupled to said first detecting means for generating agate having a duration of said predetermined time in response to thepresence of said enabling signal.

10. The apparatus of claim 8, in which said disabling signal producingmeans comprises gating means having a first input coupled to said firstdetecting means and a second input coupled to said third comparingmeans.

11. The apparatus of claim 1, further comprising frequency selectingmeans operatively interposed between said detecting means and saidanalog signal producing means.

12. The apparatus of claim 11, in which said analog level signalproducing means comprises means for comparing the detected analog signalagainst a preset reference level and to produce an enabling level signalwhen the former bears a predetermined relation to the former.

13. The apparatus of claim 1, further comprising means for deriving asynthetic signal from said detected analog signal, said sensing meanscomprising means for comparing the detected analog signal and saidsynthetic signal a predetermined time after the production of saidanalog signal, and for producing said disabling signal when the formerbears a predetermined relationship to the latter.

14. The apparatus of claim 13, in which said synthetic signal derivingmeans comprises second detecting means receiving said analog signal, andmeans for enabling said second detecting means after said analog signaldetection and 'for thereafter disabling said second detecting means atsaid predetermined time thereafter.

15. The apparatus of claim 14, in which said enabling and disablingmeans comprises means coupled to said first detecting means forgenerating a gate having a duration of said predetermined time inresponse to the presence of said level signal.

16. The apparatus of claim 15, in which said disabling signal producingmeans comprises gating means having a first input coupled to said firstdetecting means and a second input coupled to said comparing means.

17. The apparatus ofclaim 11, further comprising amplifying meansoperatively interposed between said detecting means and said frequencyselecting means.

18. The apparatus of claim 17., in which said detecting means comprisesa piezo element, and further comprising a holder having a masssignificantly greater than that of said piezo element for carrying saidpiezo element and adapted to be securely clamped to the structure beingmonitored, said amplifying-means comprising circuit elements securelymounted on said holder.

1. Apparatus for detecting and monitoring the incremental growth of acrack in a structure, comprising means for detecting the release ofacoustic energy from the structure such as that produced as a result ofa crack event in the structure, said detecting means including means forproducing an analog signal having a predetermined relation to themagnitude of the energy released upon each release of such energy, andmeans coupled to said analog signal producing means for deriving abinary signal representative of the number and amplitude of said analogsignals, said binary signal deriving means including means for producingone or more count signals the number of which bears a predeterminedrelationship to the amplitude of said analog signal, means coupled tosaid count signal producing means for accumulating said count signals,and means coupled to said detecting means for sensing the nature of thedetected analog signal between a crack event signal and an impactsignal, and means coupled to said sensing means for disabling saidbinary signal deriving means upon the determination that said detectedanalog signal is a result of an impact on the structure.
 2. Theapparatus of claim 1, further comprising means for comparing the numberof accumulated count signals against a preset reference level, alarmmeans, and means coupled to said comparing means for actuating saidalarm means when the number of accumulated counted signals equals orexceeds said reference level.
 3. The apparatus of claim 1, furthercomprising means for operatively comparing said analog signal and apreset reference level and for producing an enabling signal when theformer bears a predetermined relation to the latter.
 4. The apparatus ofclaim 3, further comprising means operatively interposed between saiddetecting means and said comparing means for operatively integratingsaid detected analog signal to thereby produce a peak signal, said peaksignal being applied to one input of said comparing means.
 5. Theapparatus of claim 3, in which said count signal producing meanscomprises pulse producing means coupled to said comparing means andactuated in response to said comparing means when said analog signalbears said predetermined relation to said reference signal.
 6. Theapparatus of claim 5, further comprising means coupled to said pulseproducing means for summing the pulses produced by the latter, secondmeans for comparing the summed number of said count signals against apreset count, and means for producing a warning signal when the formerequals or exceeds the latter.
 7. The apparatus of claim 1, furthercomprising means for deriving a synthetic signal from said detectedanalog signal, said sensing means comprising means for comparing thedetected analog signal and said synthetic signal at a predetermined timeafter the detection of said analog signal and for producing a countdisabling signal when the former bears a predetermined relationship tothe latter.
 8. The apparatus of claim 7, in which said synthetic signalderiving means comprises second detecting means receiving said analogsignal and means for enabling said second detecting means after saidanalog signal detection and for thereafter disabling said seconddetecting means at said predetermined time thereafter.
 9. The apparatusof claim 8, in which said enabling and disabling means comprising meanscoupled to said first detecting means for generating a gate having aduration of said predetermined time in response to the presence of saidenabling signal.
 10. The apparatus of claim 8, in which said disablingsignal producing means comprises gating means having a first inputcoupled to said first detecting means and a second input coupled to saidthird comparing means.
 11. The apparatus of claim 1, further comprisingfrequency selecting means operatively interposed between said detectingmeans and said analog signal producing means.
 12. The apparatus of claim11, in which said analog level signal producing means comprises meansfor comparing the detected analog signal against a preset referencelevel and to produce an enabling level signal when the former bears apredetermined relation to the former.
 13. The apparatus of claim 1,further comprising means for deriving a synthetic signal from saiddetected analog signal, said sensing means comprising means forcomparing the detected analog signal and said synthetic signal apredetermined time after the production of said analog signal, and forproducing said disabling signal when the former bears a predeterminedrelationship to the latter.
 14. The apparatus of claim 13, in which saidsynthetic signal deriving means comprises second detecting meansreceiving said analog signal, and means for enabling said seconddetecting means after said analog signal detection and for thereafterdisabling said second detecting means at said predetermined timethereafter.
 15. The apparatus of claim 14, in which said enabling anddisabling means comprises means coupled to said first detecting meansfor generating a gate having a duration of said predetermined time inresponse to the presence of said level signal.
 16. The apparatus ofclaim 15, in which said disabling signal producing means comprisesgating means having a first input coupled to said first detecting meansand a second input coupled to said comparing means.
 17. The apparatus ofclaim 11, further comprising amplifying means operatively interposedbetween said detecting means and said frequency selecting means.
 18. Theapparatus of claim 17, in which said detecting means comprises a piezoelement, and further comprising a holder having a mass significantlygreater than that of said piezo element for carrying said piezo elementand adapted to be securely clamped to the structure being monitored,said amplifying means comprising circuit elements securely mounted onsaid holder.